This invention relates to microfine silica and a method of precipitating it. It relates further to hydrophobic microfine silica modified by inclusion of polyvalent ions on the surface of the silica.
The preparation of finely divided or microfine silica, 10 to 1000 m.mu., is well known and numerous patents have issued describing such processes. An example of such a patent is U.S. Pat. No. 3,208,823, hereby incorporated by reference. This patent describes methods of preparing finely divided hydrated silica by forming a protocoacervate from a soluble silicate solution using coacervating agents such as highly soluble salts, especially univalent inorganic salts; completely water miscible hydrogen bonding agents as, for example, simple alcohols and ketones, and highly soluble nitrogen bases such as ammonia and amines. The silica prepared by this and other precipitation methods using a coacervation step, always involves a secondary, highly alkaline phase some of which will be in the surface of the coacervated micelles and can be formed into gel when the micelles are precipitated as silica. This gel structure prevents the use of such precipitated silica in certain applications.